Generally, GaN-based nitride semiconductors find its application fields in electronic devices (i.e., high-speed switching and high output devices) such as optical devices of blue/green LED (Light Emitting Diode), MESFET (Metal Semiconductor Field Effect Transistor) and HEMT (High Electron Mobility Transistors).
The GaN based nitride semiconductor light emitting device is grown on a sapphire substrate or a SiC substrate. Then, an AlYGa1-YN polycrystalline thin film is grown on the sapphire substrate or the SiC substrate as a buffer layer at a low growth temperature. Then, an undoped GaN layer, a Si-doped n-GaN layer, or a mixture of the above two structures is grown on the buffer layer at a high temperature to form an n-GaN layer. Also, a Mg-doped p-GaN layer is formed at upper layer to manufacture a nitride semiconductor light emitting device. An emission layer (a multiple quantum well structure activation layer) is interposed between the n-GaN layer and the p-GaN layer.
A related art p-GaN layer is formed by doping Mg atoms while growing crystal. It is required that Mg atoms implanted as a doping source during crystalline growth be substituted by Ga location and thus serve as a p-GaN layer. The Mg atoms are combined with a hydrogen gas dissolved in a carrier gas and a source to form a Mg—H complex in a GaN crystalline layer, resulting in a high resistant material of about 10.
Accordingly, after a pn junction light-emitting device is formed, there is an need for a subsequent activation process for cutting the Mg—H complex and substituting the Mg atoms at the Ga location. However, in the light-emitting device, the amount of carriers contributing to light emission in the activation process is about 1017/, which is very lower than the Mg atomic concentration of 1019/ or higher. Accordingly, there is a disadvantage in that it is very difficult to form a resistive contact.
Furthermore, the Mg atoms remaining within the p-GaN nitride semiconductor without being activated as carriers serve as the center at which light emitted from the interface with the active layer is trapped, abruptly decreasing the optical output. In order to improve this problem, a method in which contact resistance is lowered to increase current injection efficiency using a very thin transparent resistive metal material has been employed.
However, the thin transparent resistive metal used to decrease the contact resistance is about 75 to 80% in optical transmittance. The remaining optical transmittance serves as loss. More particularly, there is a limit to reducing an operating voltage due to high contact resistance.